Electric vehicles, that is, vehicles, in particular road vehicles, having an electromotive drive are generally known. The achievable range of such an electric vehicle is often problematic, since known stores for electrical energy, generally batteries, only have a limited storage capacity. What is known as a range extender can be used for vehicles that are driven exclusively electromotively. Said range extender is an internal combustion engine that is operated with a fuel and drives a generator for generating electrical energy. The respective electromotive drive of the vehicle and the respective battery can then be supplied with the electrical energy generated in this manner. Such a vehicle can also be referred to as a serial hybrid vehicle, since one drive, namely the internal combustion engine, is used to generate energy to supply the other drive, namely the respective electric motor, which is series-connected downstream. Such a range extender can be configured as a unit and therefore corresponds to a motor-generator unit.
Such a motor-generator unit generally comprises a piston engine with internal combustion, that is, an internal combustion engine, and a generator drive-connected thereto for electricity generation. Such a motor-generator unit can be used in a stationary manner to generate electricity, for example on building sites or in the outdoor field. Such a motor-generator unit can likewise be used in a mobile manner, namely in a vehicle. As explained above, use in an electric vehicle in order to increase the range of the electric vehicle as required is of particular interest. Such an electric vehicle comprises at least one electric motor for driving the vehicle. The electrical energy is supplied in principle by means of a battery carried in the vehicle. If this battery is completely discharged, such a motor-generator unit can be used to generate electrical power in order to ensure the electricity supply to the respective electric motor and to charge up the battery of the vehicle again. As mentioned above, such a motor-generator unit is often referred to in such mobile applications as a “range extender”.
It is of great importance precisely for such mobile applications that both the piston engine and the generator each have a comparatively low weight so that as little additional mass as possible has to be carried in the respective vehicle. A low weight for the piston engine and the generator entails low inertia masses for the piston engine and generator. This means in the case of the piston engine that each expansion stroke of a cylinder results in a significant acceleration of a crankshaft coupled to the piston. Furthermore, the crankshaft speed decreases again during the two subsequent strokes of the respective piston, namely the exhaust stroke and the induction stroke after that. During the following compression stroke, the crankshaft is also significantly slowed. Moreover, if a load is coupled to the piston engine, in particular in the form of a demand for power from the generator, a generator torque produced by the generator counteracts the engine torque and results in severe slowing of the crankshaft in crankshaft angle ranges in which there is no expansion stroke of a piston. Within a rotation cycle of the crankshaft characterised in that all the pistons connected to the crankshaft have run through each of their working strokes once, this results in a comparatively large difference in rotation speed between a maximum rotation speed and a minimum rotation speed within the respective rotation cycle.
If the crankshaft is drive-connected to a plurality of pistons, the individual acceleration and deceleration processes overlap corresponding to the respective phase shift of the pistons in relation to a crankshaft angle. This means that such a lightweight piston engine produces considerable vibrations. A highly vibrating or oscillating piston engine is however undesirable precisely for electric vehicles or vehicles having a serial hybrid drive, since such an electric vehicle runs extremely quietly during normal driving operation, during which only the at least one electric motor effects the propulsion of the vehicle. Therefore, when the piston engine of a motor-generator unit is switched on as needed, this is very often considered annoying by the users of such electric vehicles, owing to the suddenly perceptible and where applicable audible oscillations and vibrations. The users are often irritated and concerned and incorrectly assume that there is damage to the vehicle.
In order to reduce such vibrations and oscillations in lightweight piston engines, it is in principle possible to couple the crankshaft to a flywheel mass, as a result of which the inertia mass of the piston engine overall can be considerably increased. At the same time, the weight of the entire motor-generator unit increases correspondingly. The costs and the necessary installation space are also increased thereby.